Aluminum nitride sintered body

ABSTRACT

[Object] It is an object of the present invention to provide an aluminum nitride sintered body having resistance to plasma gas and high thermal conduction and having excellent optical properties. 
     [Solution means] The aluminum nitride sintered body of the present invention is characterized in that the proportion of positrons which are annihilated within a period of 180 ps (picoseconds) in the aluminum nitride crystal, as determined in the defect analysis using a positron annihilation method, is not less than 90%, and the sintered body preferably has a thermal conductivity of not less than 200 W/mK.

TECHNICAL FIELD

The present invention relates to a novel aluminum nitride sintered body.More particularly, the invention relates to an aluminum nitride sinteredbody which has excellent light transmission properties and can bepreferably used particularly as a translucent cover for a light sourceof high luminous efficiency.

BACKGROUND ART

As light transmitting materials, materials that are transparent tovisible light, such as transparent resin, glass, quartz and lighttransmitting alumina, have been heretofore properly used according tothe use environment, cost, etc. For example, as a cover (windowmaterial) for a light source of low energy intensity or as a lighttransmitting material by which light of wavelength rather including noultraviolet light is transmitted, a transparent resin or a glass isused. As a translucent cover for a light source rather includingultraviolet light or a light source having high energy intensity andthereby having a high temperature when it is used, a material usingquartz or alumina is used. As a translucent cover for a light sourceusing a corrosive gas such as a halogen gas, an alumina material havingcorrosion resistance is used.

Recently, improvement of light sources has been further promoted, andlight sources of higher luminous efficiency have been produced. Forexample, light sources using, as luminescent materials, enclosurescontaining halides (particularly iodides and bromides) of metals, suchas Na, Sc, Sn, Th, Tl, In, Li Tm, Ho and Dy, are known. Enhancement ofluminance of light sources, however, increases heat generated, so thatin the light sources of high luminous efficiency, materials of lighttransmitting members such as covers applied to the light sources becomeproblems. That is to say, although the alumina material has resistanceto halogen gas, the resistance is still insufficient. Moreover, becauseof low thermal conductivity of 30 W/mK, heat dissipation of a lightsource becomes insufficient, and therefore, there is a fear that thelife of the light source is shortened. Further, there is another problemof poor color rendering properties because the temperature of an arctube surface becomes ununiform. Accordingly, a light transmitting memberhaving resistance to halogen gas and high thermal conduction propertiesis desired.

In order to solve the above problems, aluminum nitride that is excellentin heat resistance, thermal conduction and mechanical strengthproperties has been proposed as a material of a light transmittingwindow material that transmits infrared rays or laser beam (see patentdocument 1). In this publication, it is disclosed that when a powderyraw material having a specific particle diameter of the material powder,a specific content of metallic impurities and a specific content ofoxygen is sintered in an inert atmosphere of 1700 to 2100° C., an AlNsintered body showing a transmittance of 75% in the wavelength region of0.2 μm to 30 μm is obtained.

Further, an arc tube having a translucent cover (hollow tube) composedof an aluminum nitride sintered body which is produced by the use of araw material aluminum nitride powder having such a particle sizedistribution that the amount of particles having diameters of 0.3 D to1.8 D (D: average particle diameter) is not less than 70% is disclosed(see patent document 2). In a working example of this publication, analuminum nitride sintered body having a total light transmittance of 84%is shown.

According to the techniques described above, it is possible to producean AlN sintered body improved in the light transmission properties.However, there is yet room for improvement in the light transmittance.That is to say, in the case where an aluminum nitride sintered body isused as a translucent cover, its transmittance in the visible region of400 nm to 800 nm is desired to exceed 85% taking reflectance intoconsideration, but the transmittance (400 nm to 800 nm) of an aluminumnitride sintered body obtained by the above-mentioned publicly knowntechniques is at most 85%. In contrast therewith, the alumina materialhas a transmittance exceeding 95% though it is inferior to the aluminumnitride sintered body in the resistance to halogen gas and the thermalconductivity. In comparison with the alumina material, therefore, thealuminum nitride sintered body is requested to be further improved inthe transmittance.

Moreover, with regard to the light transmission properties in theultraviolet region, there is yet room for improvement in the build-upproperties of light transmittance of the conventional light transmittingaluminum nitride sintered body. When the aluminum nitride sintered bodyis used as a translucent cover, the build-up properties of lighttransmittance are important properties to realize high lighttransmittance in the wide wavelength region of the ultraviolet region tothe infrared region.

In a patent document 3, there is disclosed an aluminum nitride sinteredbody characterized in that the oxygen concentration is held down to notmore than 400 ppm, the metallic impurity concentration is held down tonot more than 150 ppm, the carbon concentration is held down to not morethan 200 ppm, and the sintered body has an average crystal graindiameter of 2 μm to 20 μm. In this sintered body, the inclination of thespectral curve in the wavelength region of 260 to 300 nm is not lessthan 1.0 (%/nm), the light transmittance in the wavelength region of 400to 800 nm is not less than 86%, and the wavelength at which 60% of alight transmittance is reached in the spectrum is 400 nm.

Patent document 1: Japanese Patent Laid-Open Publication No. 26871/1990

Patent document 2: Japanese Patent Laid-Open Publication No. 193254/1985

Patent document 3: Japanese Patent Laid-Open Publication No. 119953/2005

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, the light transmittance (total transmittance) of the aluminumnitride sintered body specifically disclosed in a working example of thepatent document 3 is at most 87%, and this sintered body has beenimproved as compared with the conventional aluminum nitride sinteredbody, but it is inferior to the alumina material that has been alreadypractically used, so that further improvement is desired.

Accordingly, it is an object of the present invention to provide analuminum nitride sintered body having resistance to plasma gas and highthermal conduction and having excellent optical properties.

MEANS TO SOLVE THE PROBLEM

It is described in the patent documents 1 and 3 that the content ofoxygen and the content of metallic impurities in an aluminum nitridesintered body exert influence on the optical properties. An aluminumnitride sintered body is generally formed from aluminum nitride crystalgrains and a grain boundary phase. The grain boundary phase contains, asa main component, a sintering additive such as yttria. The content ofoxygen and the content of metallic impurities in the patent documents 1and 3 are values evaluated based on the whole amount of the sinteredbody, and therefore, most of the amounts of oxygen and metallicimpurities are considered to be attributable to oxygen and metals(yttrium, etc.) present in the grain boundary phase. That is to say, inthe inventions of the patent documents 1 and 3, it is designed toimprove optical properties by controlling the total composition of thesintered body including the grain boundary phase.

On the other hand, aluminum nitride crystal to constitute the aluminumnitride sintered body that is a polycrystalline body, particularlyconnection between the crystal and the optical properties, has beenhardly studied.

Under such circumstances as mentioned above, the present inventors havestudied connection between the characteristics of aluminum nitridecrystal to constitute an aluminum nitride sintered body and the opticalproperties of the sintered body, and as a result, they have found thatthere is a possibility that defects (e.g., vacancy type defects) in thealuminum nitride crystal exert influence on the optical properties ofthe sintered body. That is to say, it has been found that with increaseof the defects in the crystal, the light transmittance of the sinteredbody tends to be deteriorated. This suggests that the optical propertiesof the sintered body can be improved by decreasing the defects of thecrystal. Then, the present inventors have invented means to decrease thealuminum vacancy type defects and have accomplished the presentinvention.

The defects of the aluminum crystal grains are evaluated by a positronannihilation method.

The means to solve the above problems, which are provided by the presentinvention, are as follows.

(1) An aluminum nitride sintered body obtained from raw materialscontaining an aluminum nitride powder and a sintering additive of analkaline earth group based oxide, wherein the proportion of positronswhich are annihilated within a period of 180 ps (picoseconds) in thealuminum nitride crystal, as determined in the defect analysis using apositron annihilation method, is not less than 90%.

(2) The aluminum nitride sintered body as stated in (1), which has athermal conductivity of not less than 200 W/mK.

EFFECT OF THE INVENTION

According to the present invention, an aluminum nitride sintered bodyhaving resistance to plasma gas and high thermal conduction propertiesand having excellent optical properties is provided. Such an aluminumnitride sintered body is promising as, for example, a material of ahigh-luminance discharge lamp arc tube.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described in detail hereinafter together withits best mode.

The aluminum nitride sintered body generally comprises aluminum nitridecrystal grains and a grain boundary phase containing a sinteringadditive as a main component. The grain boundary phase, however, is notnecessarily essential, and a sintered body having no grain boundaryphase and consisting of only aluminum nitride crystal grains is alsoincluded in the present invention.

In the aluminum nitride sintered body of the invention, the proportionof positrons which are annihilated within a period of 180 ps(picoseconds) in the aluminum nitride crystal, as determined in thedefect analysis using a positron annihilation method, is not less than90%, preferably not less than 95%, more preferably not less than 98%.The upper limit is theoretically 100%, and according to the presentinvention, an aluminum nitride sintered body in which 100% ofirradiation positrons are annihilated within a period of 180 ps(picoseconds) is provided.

The positron annihilation method is a method wherein radioactive isotopesuch as ²²Na or ⁵⁸Co is used as a positron ray source, and by allowingpositrons resulted from β⁺ decay of the radioactive isotope to enter asample and by measuring a positron life up to the time of pairannihilation mainly with free electrons in the sample, vacancies ordefects in the sample are detected. The term “positron life” used hereinmeans a period of time from entering of a positron into the sample topair annihilation with an electron.

The positron is an anti-particle of an electron, and it has the samemass as that of the electron but has positive electric charge. When thepositron enters a sample, it is moderated to about thermal energy inseveral ps (picoseconds). This is referred to as a “thermalizedpositron”. The thermalized positron undergoes diffusion migration of adistance of about 10⁻⁷ m in the crystal, then undergoes pairannihilation with one of electrons in the crystal and simultaneouslyreleases an annihilation γ-ray. By detecting the annihilation γ-ray, thepositron life is measured. In case of an aluminum nitride crystal havingno vacancy type defect (perfect crystal), the positron life is about138±10 ps.

By the way, the positron has positive electric charge, so that it repelsa positive ion shell (aluminum ion) that constitutes the crystal andtries to go away from it. Therefore, if a defect where the positive ionshell falls off, such as an atomic vacancy, a microvoid(three-dimensional vacancy cluster of about 1 nm) or a void (such adefect being referred to as a “vacancy type defect” hereinafter), ispresent, the positron having reached the vacancy type defect is trappedthere (such positron being referred to as a “trapped positron”). Theelectron density in the vacancy type defect is lower than that in theperfect crystal, so that the life of the trapped positron becomes longerthan the life in the perfect crystal and exceeds usually 150 to 200 ps.

Accordingly, by measuring the positron life, the vacancy type defect ofan aluminum nitride crystal that constitutes a sintered body can beevaluated.

In case of the aluminum nitride sintered body of the invention, theproportion of positrons which are annihilated within a period of 180 ps(picoseconds) in the aluminum nitride crystal, as determined in thedefect analysis using a positron annihilation method, is not less than90%, and it can be understood that the aluminum nitride sintered body ofthe invention is consisting essentially of perfect crystal and has novacancy type defect substantially.

The aluminum nitride sintered body of the invention has a thermalconductivity of preferably not less than 200 W/mK, more preferably notless than 210 W/mK, particularly preferably not less than 230 W/mK, andalso has high thermal conduction properties that are inherentlypossessed by an aluminum nitride sintered body.

Such an aluminum nitride sintered body of the invention has excellentlight transmission properties and has a total transmittance of not lessthan 70%, preferably 70 to 90%, more preferably 90 to 98%. A method forspecifically evaluating the total transmittance is described in detailin the working example.

The aluminum nitride sintered body of the above properties has suchoptical properties as mentioned above in addition to the high thermalconduction properties and the high resistance to chemical corrosion thatare inherently possessed by aluminum nitride. Therefore, also in thecase where the aluminum nitride sintered body is applied to an arc tubeusing a light source of high luminance, lengthening of a life of thelight source can be realized.

In the case where the aluminum nitride sintered body is applied to atranslucent cover such as an ultraviolet transmitting window,realization of high ultraviolet light transmittance is possible becauseof the aforesaid optical properties.

Next, a process for producing the aluminum nitride sintered body of theinvention is described. The process for producing the aluminum nitridesintered body of the invention is not specifically restricted as long asthe aluminum nitride sintered body has the above properties.

The aluminum nitride sintered body of the invention is obtained by, forexample, heat-treating an aluminum nitride sintered body, which isobtained by a general sintering method (such a sintered body beingsometimes referred to as a “raw material sintered body” hereinafter), inthe presence of a high-temperature decomposable aluminum compound.

As the raw material sintered body, an aluminum nitride sintered bodywherein the proportion of positrons which are annihilated within aperiod of 180 ps (picoseconds) in the aluminum nitride crystal, asdetermined in the defect analysis using a positron annihilation method,is less than 90%, preferably 50 to 90%, is used. That is to say, analuminum nitride sintered body having relatively few vacancy typedefects is preferably used as the raw material sintered body.

As the raw material sintered body, any of various aluminum nitridesintered bodies is employable. One example of processes for producingvarious sintered bodies is described below, but the process employableis not limited thereto.

The raw material sintered body is produced by molding a mixture of analuminum nitride powder and a sintering additive into a molded productof a given shape and sintering the molded product in a reducingatmosphere.

As the aluminum nitride powder used as the raw material, a powder havingparticle diameters capable of attaining crystal grain diameters of 2 to20 μm by sintering is preferably used. In general, a powder having anaverage particle diameter that is a little smaller than the abovecrystal grain diameters is preferably used taking grain growth in thesintering into consideration, and for example, a powder having anaverage particle diameter of 0.5 to 15 μm, more preferably 1 to 10 μm,is preferable.

In order to hold down the concentration of impurities in the sinteredbody to a low concentration, the aluminum nitride powder is preferably ahigh-purity one having a purity of not less than 97% by weight,desirably not less than 99% by weight, and most preferably used ishigh-purity aluminum nitride wherein the metallic impurity concentration(concentration of metals other than Al) is not more than 50 ppm and theoxygen concentration has been reduced to not more than 1% by weight,particularly not more than 0.8% by weight.

Further, in order to reduce the concentration of oxygen in the sinteredbody, which is a main cause of a vacancy type lattice defect, analuminum nitride powder containing carbon as an impurity component isalso employable. That is to say, by carrying out sintering in thepresence of carbon, oxygen contained as an impurity reacts with thecarbon and is removed as a carbon dioxide gas. However, if such carbonis contained in a large amount in the raw material powder, it remains asan impurity in the sintered body and is liable to deteriorate lighttransmission properties, so that the concentration of carbon in thealuminum nitride powder is preferably not more than 450 ppm.

As the sintering additive, a publicly known one, for example, analkaline earth group based oxide, such as CaO, SrO or Ca₃Al₂O₆, isemployed. Further, a rare earth group based oxide, such as Y₂O₃, CeO₂,Ho₂O₃, Yb₂O₃, Gd₂O₃, Nb₂O₃, Sm₂O₃ or Dy₂O₃, is also employable. However,it is preferable to use the rare earth group based oxide in such amanner that the content of rare earth elements in the aluminum nitridesintered body finally obtained becomes less than 0.01 ppm, and it ismore preferable to use it in such a manner that any rare earth elementis not contained at all. The sintering additive does not necessarilyhave to be an oxide, and it may be, for example, a carbonate, a nitrateor a phosphate. The amount of the sintering additive added is preferablyin the range of 2 parts by weight to 20 parts by weight based on 100parts by weight of the aluminum nitride powder. If the amount thereof issmaller than 2 parts by weight, high purity of the aluminum nitridesintered body cannot be realized, and properties such as lighttransmittance and thermal conductivity are lowered. Also in the casewhere the amount of the sintering additive exceeds 20 parts by weight,the sintering additive added cannot volatilize efficiently, andproperties such as light transmittance and thermal conductivity arelowered.

Mixing of the aluminum nitride powder with the sintering additive powdercan be carried out by a publicly known method. For example, mixing by adry process or a wet process using a mixing machine such as a ball millis preferably adoptable. In the wet mixing method, dispersion media suchas alcohols and hydrocarbons are used. From the viewpoint ofdispersibility, it is preferable to use alcohols or hydrocarbons.

For the mixing, the sintering additive is preferably stored in dry airso as not to undergo water absorption or aggregation and if necessaryvacuum dried, and it is preferable to immediately mix a powder of thesintering additive with the aluminum nitride powder.

Prior to sintering, the mixed powder is molded into a molded product ofa given shape according to the use purpose, and molding can be carriedout by a publicly known method. In order to mold the powder into amolded product of high strength and in order to enhance yields, moldingis preferably carried out using an organic binder.

For example, the mixed powder is mixed with an organic binder and ifnecessary with a dispersant, a plasticizer, a solvent, etc. to prepare amolding slurry or pastep. This molding slurry or pastep is molded by amolding method, such as a doctor blade method, an extrusion moldingmethod, an injection molding method or a cast molding method, whereby amolded product can be produced. Examples of the organic binders includebutyral resins such as polyvinyl butyral and acrylic resins such aspolybutylmethacrylate. Such an organic binder can be used in an amountof 0.1 to 30 parts by weight, particularly 1 to 15 parts by weight,based on 100 parts by weight of the aluminum nitride powder. Examples ofthe dispersants include glycerol compounds. Examples of the plasticizersinclude phthalic esters. Examples of the solvents include isopropylalcohol and hydrocarbons.

It is also possible to carry out molding by a compression molding methodwithout using an organic binder. For example, from a mixed power of thealuminum nitride powder and the sintering additive powder, a temporarymolded product is produced by the use of a monoaxial molding machine,and the temporary molded product is pressured-molded at 1 to 4 t/cm² bythe use of a CIP (cold isotactic press) molding machine, whereby amolded product can be produced.

The resulting molded product is degreased (removal of binder) and thensintered.

Although the degreasing can be carried out by heating the molded productin an arbitrary atmosphere such as air, nitrogen or hydrogen, it ispreferable to carry out degreasing in nitrogen in which control of theamount of residual carbon is easily made. The degreasing temperature isin the range of generally 300 to 900° C., particularly preferably 300 to700° C. In the case where molding is carried out without using anorganic binder as in the compression molding method, this degreasingstep is unnecessary.

In order to efficiently carry out removal of the sintering additive toreduce the concentration of metallic impurities and the concentration ofoxygen in the sintered body, sintering is carried out in a reducingatmosphere.

For realizing the reducing atmosphere, a method of allowing a carbongeneration source to be present together with the molded product in acontainer for sintering, a method of using a container made of carbon asa container for sintering, etc. are employable. Of these, the method ofallowing a carbon generation source to be present together with themolded product in a container for sintering is preferable taking thermalconductivity and color ununiformity of the resulting sintered body intoconsideration. In order to particularly obtain high thermalconductivity, a method wherein a closed container is used as thecontainer for sintering and the molded product and the carbon generationsource are placed in this closed container is most preferable.

The carbon generation source is not specifically restricted, and carbonin a publicly known state, such as amorphous carbon or graphite, isemployable. Preferable is solid carbon. The form of the carbon is notspecifically restricted, and any form of a powder, a fiber, a felt, asheet and a plate is employable. A combination of those forms is alsoemployable. Of these, amorphous carbon or graphite in a plate form ispreferable taking acquisition of high thermal conductivity intoconsideration.

The method of placing the molded product and carbon in a container isnot specifically restricted, and the carbon and the molded product maybe placed in any of a non-contact state and a contact state. Of these,the non-contact state is preferable from the viewpoint of easy controlof thermal conductivity of the resulting sintered body. As thenon-contact state, a publicly known non-contact state is adoptable, andfor example, a method of simply providing a space between carbon and themolded product, a method of interposing a powder of boron nitride or thelike between carbon and the molded product to make them be in thenon-contact state, or a method of placing a plate or the like made ofceramic such as aluminum nitride or boron nitride between carbon and themolded product to make them be in the non-contact state is employable.Taking improvement of thermal conductivity into consideration, themethod of placing a plate or the like between carbon and the moldedproduct to make them be in the non-contact state is preferable, and inorder to obtain a raw material sintered body having much higher thermalconductivity, a method of placing a plate so as to isolate a space wherecarbon is placed as much as possible from a space where the moldedproduct is placed in a closed container is particularly preferable.

The sintering in a reducing atmosphere is preferably carried out at atemperature of 1500 to 2000° C. for at least 3 hours, particularly atleast 10 hours. If the sintering is carried out for a long period oftime, growth of crystal grains of the aluminum nitride sintered body isbrought about, and besides, the concentration of carbon in the sinteredbody is increased, so that the time of sintering in a reducingatmosphere is preferably not more than 200 hours, particularlypreferably not more than 100 hours, most preferably not more than 50hours.

In order to surely decrease the carbon concentration in the sinteredbody to be within the aforesaid range, it is preferable to carry outsintering in a neutral atmosphere in combination with the aforesaidsintering in a reducing atmosphere. For example, an embodiment whereinafter sintering in a neutral atmosphere, sintering in a reducingatmosphere is carried out, or an embodiment wherein after sintering in aneutral atmosphere, sintering in a reducing atmosphere is carried outand then sintering in a neutral atmosphere is further carried out ispreferably adopted. The reason is that if sintering in a reducingatmosphere is carried out for a long period of time, the carbonconcentration is increased even if the metallic impurity concentrationis held down to be within the aforesaid range, and the opticalproperties of the sintered body are eventually deteriorated. Therefore,by limiting the time of sintering in a reducing atmosphere to theaforesaid range and by properly carrying out sintering in a neutralatmosphere, a dense and highly strong sintered body can be obtained.

The “neutral atmosphere” means an atmosphere wherein oxygen “O₂” andcarbon “C” are not substantially present, and more specifically, itmeans an atmosphere of an inert gas such as nitrogen or argon. Sinteringin a neutral atmosphere is carried out by, for example, purging a closedcontainer with an inert gas. As the closed container, a container madeof ceramic, such as aluminum nitride or boron nitride, or a non-carbonmaterial, such as tungsten “W” or molybdenum “Mo”, is employed, and fromthe viewpoint of durability, a container made of ceramic such asaluminum nitride or boron nitride is preferable. Further, a containermade of carbon whose inner surface has been coated with a material thatis the aforesaid non-carbon material and is impermeable to gasses isalso employable. In short, sintering has only to be carried out in sucha state that a carbon source other than the residual carbon in themolded product is not allowed to be present in the space of the closedcontainer.

The temperature of sintering in such a neutral atmosphere as above ispreferably in the range of 1500 to 1900° C., and the sintering time isin the range of usually 3 to 100 hours, preferably 30 to 50 hours,though it varies depending upon the time of sintering in a reducingatmosphere

Subsequently, the resulting raw material sintered body is heat-treatedin the presence of a high-temperature decomposable aluminum compound,whereby an aluminum nitride sintered body of the invention is obtained.The high-temperature decomposable aluminum compound is preferably amaterial which is stably present in the intermediate stage of sinteringof aluminum nitride and further also in the latter stage thereof andliberates an aluminum based gas into a gas phase. That is to say, amaterial which is stably present at a temperature of not lower than1000° C. and liberates an aluminum based gas is preferable. For example,Al₂O₃, Al₂S₃, AlF₃ or AlN is employable. Differently from the rawmaterial sintered body, aluminum nitride used as the high-temperaturedecomposable aluminum compound slowly liberates an aluminum based gas ata temperature of about (1500)° C. The properties of slow gas liberationof the high-temperature decomposable aluminum nitride are thought to beattributable to composition or structure of the boundary grain phase.The high-temperature decomposable aluminum compound may be in any form,such as a powder, a molded product or a sintered body, and the sameeffect is obtained also by exposing a gasified aluminum based compoundto the raw material sintered body. In the annealing step, a N₂ gas isallowed to flow under the conditions of 0.1 to 30 liters/min. Theannealing is carried out by allowing the high-temperature decomposablealuminum compound to co-exist in a sintering container made of amaterial such as dense carbon, boron nitride or aluminum nitride at anannealing temperature of 1600 to 2000° C. for 1 to 200 hours.

Without wishing to be bound by theory, it is considered that by virtueof such a heat treatment as mentioned above, the vacancy type defects inthe raw material sintered body are complimented by aluminum to formaluminum nitride crystal grains of perfect crystal or close thereto,whereby optical properties such as light transmission properties areimproved.

Since the aluminum nitride sintered body obtained as above impartssufficient strength to a device, so that it is used in various forms,such as tubular form, plate form, curved surface form, spherical form,elliptic spherical form, cup form and bowl form, according to thestructure of use purpose such as a translucent cover.

INDUSTRIAL APPLICABILITY

The aluminum nitride sintered body of the invention has excellentoptical properties such as light transmission properties in addition tothe high thermal conduction properties and the high resistance tochemical corrosion which are inherently possessed by aluminum nitride.Therefore, the sintered body can be used as a material of an arc tube ofa high-luminance light source and can realize lengthening of a life ofthe light source.

EXAMPLES

The present invention is further described with reference to thefollowing examples, but it should be construed that the invention is inno way limited to those examples.

In the examples and the comparative examples, various properties weremeasured in the following manner.

(1) Life of Positron

As a positron generation source, ²²Na was used. By β⁺ decay of ²²Na, a γray (γ₀) of 1275 keV was released together with positrons. The time atwhich the positrons were generated was confirmed by detecting γ₀ bymeans of a scintillator. Then, aluminum nitride was irradiated with thepositrons of at most 540 keV thus generated. In the aluminum nitride,the positrons were moderated to about thermal energy and then underwentpair annihilation with electrons to release 2 annihilation γ rays (γ₁)of 511 keV. The γ₁ released from the aluminum nitride was detected todetermine the time at which the positrons were annihilated. By measuringthe time difference using a time-measuring circuit, a positron lifespectrum was obtained.

In case of an aluminum nitride crystal having no vacancy type defect(perfect crystal), the positron life is about 138±10 ps. In theirradiation of the aluminum nitride sintered body with the positrons,the proportion of positrons annihilated within a period of 180 ps(picoseconds) was determined.

(2) Thermal Conductivity

Thermal conductivity was measured by a laser flash method using athermal constants measuring device PS-7 manufactured by Rigaku DenkiCo., Ltd. Correction of thickness was carried out by the use of acalibration curve.

(3) Light Transmittance

In the measurement of a light transmittance of an aluminum nitridesintered body, the aluminum nitride sintered body was processed into ashape having a diameter of 30 mm and a thickness of 0.3 mm, and itslight transmittance was measured by the use of “HZ-1” manufactured bySuga Test Instrument Co., Ltd.

(4) Spectrum

In the measurement of a spectrum of an aluminum nitride sintered body inthe wavelength region of 240 to 800 nm, the aluminum nitride sinteredbody was processed into a shape having a diameter of 30 mm, a thicknessof 0.3 mm and an average surface roughness Ra (JIS B 0601) of not morethan 0.05 μm, and its spectrum was measured by the use of “UV-2100”manufactured by Shimadzu Corporation. From the spectral curve, aninclination (build-up properties) in the wavelength region of 260 to 300nm and a wavelength at which 60% of a light transmittance was reachedwere determined.

Example 1

In a nylon pot having an internal volume of 2.4 liters, nylon ballswhose iron cores had been coated with nylon and each of which had adiameter of 15 mm (surface hardness: not more than 100 kgf/mm², density:3.5 g/cm³) were placed, then 100 parts by weight of an aluminum nitridepowder having an average particle diameter of 1.3 μm, a specific surfacearea of 3.39 m²/g, an oxygen concentration of 0.8 wt % and a metallicelement concentration of 35 ppm, 2 parts of a calcium aluminate compound(Ca₃Al₂O₆) having an average particle diameter of 1.8 μm and a specificsurface area of 3.75 m²/g as a sintering additive powder and 40 parts byweight of ethanol as a solvent were added, and they were mixed by a wetprocess. In this mixing operation, the nylon balls occupied 40%(apparent volume) of the internal volume of the pot. The mixing wascarried out for 3 hours by rotating the pot at a rotational speed of 70rpm. The resulting slurry was dried to obtain an aluminum nitridepowder.

Subsequently, 10 g of the resulting aluminum nitride powder wassubjected to temporary molding by the use of a monoaxial molding machineto prepare a molded product having a diameter of 40 mm and a thicknessof 6 mm. Thereafter, the molded product was subjected to main molding bythe use of a CIP molding machine under application of a load of 3 t/cm².

The molded product obtained through the above operations was sintered ata sintering temperature of 1880° C. for 30 hours in a gas atmospherecontaining nitrogen and a reducing substance by the use of an aluminumnitride setter to obtain a sintered body having a diameter of 30 mm anda thickness of 5 mm. The resulting sintered body was placed in analuminum nitride setter containing 3 g of an alumina powder as ahigh-temperature decomposable aluminum compound, and then annealing wascarried out at a temperature of 1880° C. for 30 hours to obtain analuminum nitride sintered body. The conditions for producing thealuminum nitride sintered body and the properties of the resultingaluminum nitride sintered body are set forth in Table 1.

Example 2

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 3 parts. The conditions forproducing the aluminum nitride sintered body and the properties of theresulting aluminum nitride sintered body are set forth in Table 1.

Example 3

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 5 parts. The conditions forproducing the aluminum nitride sintered body and the properties of theresulting aluminum nitride sintered body are set forth in Table 1.

Example 4

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 7 parts. The conditions forproducing the aluminum nitride sintered body and the properties of theresulting aluminum nitride sintered body are set forth in Table 1.

Example 5

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 10 parts. The conditions forproducing the aluminum nitride sintered body and the properties of theresulting aluminum nitride sintered body are set forth in Table 1.

Example 6

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 5 parts and the type of thehigh-temperature decomposable aluminum compound in the annealingtreatment was changed to aluminum sulfide. The conditions for producingthe aluminum nitride sintered body and the properties of the resultingaluminum nitride sintered body are set forth in Table 1.

Example 7

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 5 parts and the type of thehigh-temperature decomposable aluminum compound in the annealingtreatment was changed to aluminum fluoride. The conditions for producingthe aluminum nitride sintered body and the properties of the resultingaluminum nitride sintered body are set forth in Table 1.

Example 8

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 5 parts and the type of thehigh-temperature decomposable aluminum compound in the annealingtreatment was changed to AlN. The high-temperature decomposable aluminumcompound used in the annealing treatment was SH30 (aluminum nitridesintered body available from Tokuyama Corporation). The conditions forproducing the aluminum nitride sintered body and the properties of theresulting aluminum nitride sintered body are set forth in Table 1.

Comparative Example 1

The same procedure as in Example 1 was repeated except that thesintering additive was not added. The conditions for producing thealuminum nitride sintered body and the properties of the resultingaluminum nitride sintered body are set forth in Table 1.

Comparative Example 2

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 0.5 part. The conditions forproducing the aluminum nitride sintered body and the properties of theresulting aluminum nitride sintered body are set forth in Table 1.

Comparative Example 3

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 1 part. The conditions forproducing the aluminum nitride sintered body and the properties of theresulting aluminum nitride sintered body are set forth in Table 1.

Comparative Example 4

The same procedure as in Example 1 was repeated except that the amountof the sintering additive was changed to 5 parts and the annealingtreatment was not carried out. The conditions for producing the aluminumnitride sintered body and the properties of the resulting aluminumnitride sintered body are set forth in Table 1.

Comparative Example 5

The same procedure as in Example 1 was repeated except that as thesintering additive, Y₂O₃ was added in an amount of 5 parts, sinteringwas carried out at a sintering temperature of 1780° C. for a retentiontime of 5 hours in a neutral atmosphere, and the annealing treatment wasnot carried out. The conditions for producing the aluminum nitridesintered body and the properties of the resulting aluminum nitridesintered body are set forth in Table 1.

Comparative Example 6

The same procedure as in Example 1 was repeated except that thesintering additive was not added, sintering was carried out at asintering temperature of 1880° C. for a retention time of 5 hours in aneutral atmosphere, and the annealing treatment was not carried out. Theconditions for producing the aluminum nitride sintered body and theproperties of the resulting aluminum nitride sintered body are set forthin Table 1.

TABLE 1 Production process Sintering additive Sintering AmountTemperature Time No. Type (part (s)) Molding (° C.) (hours) AtmosphereExample 1 C3A 2 CIP 1880 30 reducing N₂ 2 C3A 3 CIP 1880 30 reducing N₂3 C3A 5 CIP 1880 30 reducing N₂ 4 C3A 7 CIP 1880 30 reducing N₂ 5 C3A 10CIP 1880 30 reducing N₂ 6 C3A 5 CIP 1880 30 reducing N₂ 7 C3A 5 CIP 188030 reducing N₂ 8 C3A 5 CIP 1880 30 reducing N₂ Annealing TemperatureTime No. (° C.) (hours) Additive Example 1 1880 30 Al₂O₃ 2 1880 30 Al₂O₃3 1880 30 Al₂O₃ 4 1880 30 Al₂O₃ 5 1880 30 Al₂O₃ 6 1880 30 Al₂S₃ 7 188030 AlF₃ 8 1880 30 AlN Properties Proportion of Thermal Total annihilatedconductivity transmittance Build-up properties of No. positrons (%)(W/mK) (%) spectrum Example 1 97 202 86.0 1.30 2 100 216 86.2 1.35 3 100211 86.3 1.37 4 100 216 86.4 1.39 5 100 234 86.5 1.39 6 100 205 86.51.33 7 100 210 86.7 1.32 8 100 207 86.6 1.39 Production processSintering additive Sintering Amount Temperature Time No. Type (part (s))Molding (° C.) (hours) Atmosphere Compar- 1 none 0 1880 30 reducing N₂ative 2 C3A 0.5 1880 30 reducing N₂ Example 3 C3A 1 1880 30 reducing N₂4 C3A 5 CIP 1880 30 reducing N₂ 5 Y₂O₃ 5 1780 5 neutral N₂ 6 none 0 CIP1880 5 neutral N₂ Annealing Temperature Time No. (° C.) (hours) AdditiveCompar- 1 1880 30 Al₂O₃ ative 2 1880 30 Al₂O₃ Example 3 1880 30 Al₂O₃ 45 6 Properties Proportion of Thermal Total annihilated conductivitytransmittance Build-up properties of No. positrons (%) (W/mK) (%)spectrum Comparative 1 58 102 36.0 0.25 Example 2 74 150 78.0 0.70 3 82176 79.0 0.95 4 88 190 83.0 0.98 5 80 173 43.0 0.50 6 50 80 33.0 0.20C3A = Ca₃Al₂O₆ CIP = Cold Isostatic Press

1. An aluminum nitride sintered body obtained from raw materialscontaining an aluminum nitride powder and a sintering additive of analkaline earth group based oxide, wherein the proportion of positronswhich are annihilated within a period of 180 ps (picoseconds) in thealuminum nitride crystal, as determined in the defect analysis using apositron annihilation method, is not less than 90%.
 2. The aluminumnitride sintered body as claimed in claim 1, which has a thermalconductivity of not less than 200 W/mK.